1. Field of the Invention
The invention relates to a semiconductor device and a method for manufacturing the same. In particular, it relates to a structure of a field effect transistor having a submicron channel length and a manufacturing method for the same.
2. Description of the Related Art
A conventional field effect transistor having a submicron channel length (for example, MOS transistor including those with an LDD structure) has a drawback in that a threshold voltage and a source/drain breakdown voltage are lowered owing to a short channel effect. It causes a strict limitation for minimizing the size of a semiconductor device. In order to overcome the drawback, a method disclosed in U.S. Pat. No. 5,218,221 to Okumura (corresponding to Japanese Laid-Open Patent Application 3(1991)-204940) was developed, in which impurity ions are implanted at a predetermined tilt angle to the normal line of the semiconductor substrate, while rotating the substrate about the normal line thereof.
According to the above method, a device can be formed with a channel in which impurity ions are unevenly implanted. More specifically, the impurity concentration distribution in the vicinity of source and drain of the channel is locally higher than the center region thereof. Therefore, a threshold voltage of the device can be controlled. In addition, since this impurity ion implantation inhibits an extension of a depletion layer between the source and drain, the control of the breakdown voltage is also improved.
The inventors of the present invention measured a short channel effect by implanting impurity ions for controlling the threshold voltage in an oblique direction at 30.degree.. The results are shown in FIG. 10. As is seen from FIG. 10, the decrease of the threshold voltage (Vth) is improved when impurity ions are implanted in a higher dose.
Similarly, FIG. 11 shows a short channel effect by implanting impurity ions for controlling source/drain breakdown voltage in an oblique direction at 7.degree., which is a different tilt angle from that of the above measurement shown in FIG. 10.
It can be seen that the implantation at an angle of 30.degree. improves the decrease of the threshold voltage more effectively than the case of implantion at an angle of 7.degree..
As shown in FIG. 14, the conventional example shows implanting impurity ions at 30.degree. with a concentration of 7.times.10.sup.12 ions/cm.sup.2, and the voltage abruptly goes down when having a short channel length, for example shorter than 0.5 .mu.m. However, even when implanting impurity ions at the same angle used for FIG. 11, such a lowering of the voltage does not occur in a short channel region if the implantation is conducted with 1.times.10.sup.13 ions/cm.sup.2.
As is apparent from FIGS. 10 and 11, the preferred dose of ion implantation for controlling threshold voltage is different from that for controlling the source/drain breakdown voltage.
However, according to the method described above, as p level ions are implanted at one type of concentration by the oblique ion implantation, only the ion impurity region 11 is formed as shown in a cross sectional view of a conventional semiconductor in FIG. 16. Therefore, it was not possible to satisfy the two conditions simultaneously i.e., a condition for controlling the short channel effect and a condition for improving the breakdown voltage by inhibiting the extension of the depletion layer between the source and drain, in order to obtain the preferred condition for improving the conventional defect.